This invention relates to a prestressed, post-tensioned concrete pressure-containment vessel which contains one or more cavities subjected to unusually high pressures within the confinement of the prestressing force. It also relates to a method for constructing such a vessel, which may be according to new design criteria which provide for the containment of higher internal pressures and temperatures.
Pressure-containment vessels for pressures up to the order of 1,000 p.s.i. and using prestressed concrete construction have been constructed for various purposes such as nuclear power containment, water, and oil tanks. Usually there has been only a single cavity contained by the external vessel walls, although some such vessels have included several cavities. One aspect of the present invention centers on a provision of more than one cavity within a container and a relationship such that internal pressures exerted by the various cavities neutralize one another, thereby making the vessel and the entire containment much more economical. It also includes the addition of internal tendons or bars, which further strengthen the local conditions as required.
An important object of the present invention is to provide pressure containment vessels which can withstand high pressures in large multiple cavities, which would otherwise have to be built and contained separately.
Another important object is to provide a method for prestressing and reinforcing such vessels, both externally and internally, that will significantly reduce the cost of these structures.
Another object is to provide a vessel which will contain the various pressure cavities required for the processing of energy production and materials conversion, so that the heat and energy loss can be minimized by virtue of having all the heat exchange contained within a single vessel, thereby enabling the efficient tapping of heat energy, as by a cooling system associated with the vessel, to be utilized as a source of power.
Another object is to effectively increase the pressure and temperature-retaining capability of a precast concrete vessel by using a new set of design criteria which include the encouragement and actual inducement of cracking, on a controlled basis, at preselected locations in the outer portions of the thick vessel wall.
A concrete vessel of this invention may be constructed from either precast concrete or in-place concrete, or from a combination thereof. It is totally contained by prestressing tendons that apply forces containing the pressures within the vessel. It may be additionally reinforced with internal post-tensioned tendons, as well as reinforcing bars. Inside the vessel is a plurality of cores or cavities, which may be of cylindrical or any other form and which are contained by the overall external prestressing, with the help of the internal tendons and/or reinforcements, preferably of steel.
The form of the concrete vessel may be spherical, cylindrical, octagonal, hexagonal, rectangular or any other shape or combination. The ends of the concrete vessel can be flat, semi-spherical, ellipsoidal, parabaloidal, pyramidal, or any other shape or combinations, so as to meet the requirement of functional capabilities and prestressed tendon arrangement. The number, size, location, and shape of the hollow cores can be varied at will, and they may be connected by necessary piping or tunnels within the vessel, as desired.
A multiple-cavity integrated pressure vessel according to the invention has several essential features.
First, the pressures of the various hollow cores inside the vessel are utilized to act against one another and, therefore, to neutralize their forces so that only one global set of prestressed tendons is required around the outside of the concrete vessel. The concrete casing under compression absorbs the pressure from the tendons when the cores are not pressurized, and that compression is partly released when resisting internal pressure. In order to control internal stresses produced by irregular patterns of arrangement of the interior cores, additional prestressed tendons or non-prestressed reinforcements can be located within the vessel, terminating either within the vessel or at the exterior facade of the vessel.
A second basic feature of this integrated vessel is the simplification, and hence the economy, of the piping and other connections required to connect the cores. By locating all these cores and their piping within one vessel, they become cylinders or tunnels in the vessel, and need not to be separately contained for pressure-resisting. The multiple areas of interface between the external atmosphere and between the individual cores, pipes, and connections, are thereby reduced to a single facade around the periphery of the entire concrete vessel or block.
The concrete block forming the vessel may be prestressed and confined in three dimensions, so that the strength of its concrete is increased by three to five times the ordinary unconfined strength of concrete. For example, concrete having a normal strength of 5000 p.s.i. can, in this invention resist a compressive stress of 20,000 p.s.i. In fact, it is difficult to visualize an ultimate failure of concrete within this exterior confinement. It is understood that the plastic flow of concrete within that block (when subject to extremely high compressive stresses) must be taken into account.
The basic weakness of concrete in tension can be overcome, according to one aspect of the present invention, by three-dimensional precompression produced by the tendons. This three-dimensional precompression is in sharp contrast to the one or two-dimensional precompression usually obtained for single-cavity prestressed concrete vessels.
The design and analysis of such a vessel subjected to external prestressing by the tendons, by internal pressure from the cores, and by effects of additional steel within the vessel should be accomplished by careful physical and mathematical analyses. Such analyses, using available general computer programs or particular computer programs, should take into account the shrinkage and creep effects, as well as the elastic and plastic behavior of concrete and steel subject to the given environmental and artificially produced conditions. The effects of temperature changes, resulting in temperature gradient and material property alterations of the concrete and steel result in resisting such effects as are accounted for by the external prestressing, and this is a tremendous asset to that design.
In order to control the heat and heat transfer of the thermal reactions within the cores, it may be necessary or advisable to install cooling or heating systems around the cores and in fact throughout the vessel as required. The heat gain or loss from these thermal control systems may then be utilized to produce energy within or outside the vessel.
Access to the various cores and pipes within the vessel, for the purpose of inspection and repair, can be suitably located so as to minimize stress concentration and to facilitate maintenance and operation. Such accesses will be reinforced and strengthened as required.
Lining of the cores and pipes for various reasons, such as controlling penetration, refraction or temperatures, can be provided by metals, ceramics, or plastics. But the pressure-containing effect is essentially provided by the exterior prestressing and by the additional concrete or steel within the core, utilizing the large increase in concrete strength when concrete is so confined.
The applications of these integrated vessels are specially desirable for energy-producing or converting systems, including oil, gas, coal, nuclear, solar, geothermal, and others. The exterior appearance of these concrete vessels can be suitably decorated, or they can be sunk partly or totally into the ground, ocean, or other water, as may be the case. In fact, when sunken in deep water, the external waterhead can be utilized as part of the global post-tensioning.
An important feature of a prestressed concrete pressure vessel (PCPV) according to a second embodiment of the invention is to overcome the inherent weakness of concrete in tension, by permitting and actually inducing controlled tension cracking. The tension cracks are introduced where the structural integrity of the PCPV is not impaired, i.e., the external regions of the vessel wall. The vessel wall is generally cast considerably thicker for this type PCPV. For a wall thickness of ten feet or more, the penetration of cracks to about half the wall thickness, for example, will not be detrimental in the function, safety or durability of the PCPV for the purpose for which it is designed. A PCPV designed according to this second embodiment of the invention is capable of containing higher pressure, under higher temperature, than previous PCPV's because of the relieving of many stresses due to the permitted, controlled cracking and because of the manner in which the prestressing tendons are arranged within the vessel.